1. Field of the Invention
The present invention relates to an optical beam splitting method and an optical beam splitting/modulation method which are necessary in cases where, for example, halftone plate duplicate images are recorded on a recording material by controlling a light-exposure means on its recording side in accordance with image signals obtained by photoelectric manipulation of an original image, and especially in cases where halftone plate images are recorded by independently modulating a multiple number of beams on the basis of image signals.
2. Prior Art
Recording of halftone plate images by the relative scanning of a multiple number of light beams (lined up in a row) across the surface of a recording material, with the light beams independently modulated on the basis of image signals, is conventionally known in the art. In most cases, the multiple number of light beams are obtained by installing a multiple number of totally reflective mirrors and semi-reflective mirrors, and splitting a single light beam generated by an argon laser by reflecting the light beam from the mirrors. The respective light beams thus obtained are independently modulated by means of a multi-channel ultrasonic modulator and are then reduced in diameter by means of a crystal optical system and directed onto the surface of the recording material.
If, in a case were some crosstalk is permissible in modulation in an ultrasonic modulator, the beam diameter of the Gauss beams is, for example, 1.2 mm, then the spacing of the acoustic electrodes and the spacing of the beams is set at 1.1 mm, the acoustic electrodes have a diameter of 1.2 mm and are installed in two zig-zag rows on the side surface of the modulator. Furthermore, immediately after passing through the ultrasonic modulator, the beams are reduced in diameter by means of a crystal optical system and are directed onto the surface of a recording material.
However, in order to prevent the modulation of one light beam from causing crosstalk with adjacent light beams on both sides, it is necessary to separate the beam diameters of the Gauss beams passing through the ultrasonic modulator (i.e., the beam diameters which represent an intensity distribution effective in exposing the recording material) so that there is no overlapping. For example, if the beam diameter of the Gauss beams is 1.2 mm, the acoustic electrodes are constructed with a diameter of 1.5 mm, and the spacing of the acoustic electrodes and the spacing of the beams is set at 2.0 mm. In the case of a multiple number of optical beams which are lined up in a row with widened spacing in this manner, the spacing is narrowed using optical fibers, etc., so that adjacent light beams overlap slightly with each other, and the beams are reduced in diameter by means of a crystal optical system before being directed onto the surface of a recording material.
In such a case, the light beams which are independently modulated on a beam by beam basis by the ultrasonic modulator cannot be immediately reduced in diameter by the crystal optical system and caused to expose the recording material. It is necessary first to narrow the beam spacing using optical fibers or mirrors, etc., so that the circles and the Gauss diameters of the adjacent light beams overlap slightly, forming a connected chain. Afterward, the beam diameters are reduced by a crystal optical system, and the beams are directed onto the surface of the aforementioned recording material. The reason for this is that if a multiple number of light beams lined up in a row are separated from each other instead of being overlapped in the form of a connected chain, the row of dots formed on the surface of the recording material will be a row of unconnected dots. In such a case, halftone images of various sizes cannot be formed on the surface of the recording material even if the multiple number of light beams are independently modulated on the basis of image signals.
Conversely, if the overlapping between adjacent light beams is excessive, then, as was described above, crosstalk is generated in the independent modulation of the respective light beams by the multi-channel ultrasonic modulator. Specifically, the acoustic electrodes will not only drive their corresponding light beams, but will also drive the light beams located on both sides of their corresponding light beams.
However, in cases where an optical beam splitter of the type described in Japanese Patent Application Laid-Open (Kokai) No. 52-122135 is used, the generation of a certain amount of crosstalk in the modulation occurring in the ultrasonic modulator must be tolerated. If it is desired to allow no crosstalk, it is necessary to install optical fibers, etc., after the ultrasonic modulator, and to use these to cause slight overlapping of the light beams with narrowed spacing, after which the beams are reduced in diameter by a crystal optical system and directed onto the surface of a recording material. As a result, a larger amount of space is required.
On the other hand, in cases where an optical beam splitter of the type described in Japanese Patent Application Laid-Open (Kokai) No. 58-10713 is used, a multiple number of light beams lined up in a row, which have been split into spaced light beams by the beam splitter, can be immediately reduced in diameter by means of a crystal optical system and caused to expose a recording material after being independently modulated on a beam by beam basis by a multi-channel ultrasonic modulator. Accordingly, this system is superior to that described in Japanese Patent Application Laid-Open (Kokai) No. 52-122135. The reason for this is that the beam spacing is gradually narrowed as the multiple number of light beams leave the optical beam splitter as described above. However, some problems remain when the light beams are independently modulated on a beam by beam basis by means of a multi-channel ultrasonic modulator. Specifically, since the light beams are not parallel to each other, the modulating efficiency of the modulator drops. Furthermore, light leakage may occur in some cases, and formation of the acoustic electrodes of the modulator is also difficult. In addition, maintaining the thickness and angle of intersection of the two surfaces of the optical beam splitter at prescribed values with an ultra-high degree of precision is difficult, and the manufacture of the optical beam splitter is very difficult. If the thickness or angle of intersection of the two surfaces of the optical beam splitter shows even a slight variation, the focal distances of the respective light beams will show a large variation, and the spacing between the optical beam splitter and the modulator, as well as the spacing between the modulator and the crystal optical system modulator will be unavoidably different in each individual apparatus.
Furthermore, in both the optical beam splitters described in Japanese Patent Application Laid-Open (Kokai) Nos. 52-122135 and 58-10713, the practical limit of the number of light beams into which one light beam can be split is around 20. Accordingly, it is impossible to improve the image resolution by increasing the number of split beams beyond this number. If the number of split light beams is increased beyond about 20, heat accumulates in the interior of the modulator crystal so that there are problems in terms of insufficient heat resistance and durability. Furthermore, in the case of the optical beam splitter described in Japanese Patent Application Laid-Open (Kokai) No. 52-122135, an increase in the number of split beams makes it necessary to increase the size of the crystal optical system.